The present invention relates to a method of making a self-tapping thread-forming screw starting from a blank including a right cylindrical shank portion and a tapered frusto-conical shaft portion at a work entering end of the shank portion.
Conventionally, generally three recesses are formed on the shank portion and the frusto-conical shaft portion, the recesses are also inclined to form generally triangular shape. An example of such a screw is shown in U.S. Pat. No. 3,180,126. Thread forming on the blank shown in the patent is very difficult as conventional thread rolling processes cannot be utilized.
The most relevant self tapping thread forming screw is shown in Japanese Patent Application Publication No. 31757/1979, assigned to the same assignee as is the present invention. The known thread forming screw shown in that publication is shown in FIGS. 1-4 of the present application.
Referring to FIG. 1, a screw blank 1 includes a right cylindrical shank portion 2, a head portion 3 at the rear end of the shank portion 2 having a screw driver engage recess, and a frusto-conical shaft portion at the work entering end of the shank portion 2. Three equiangularly spaced flat portions 5 are formed extending from the tip end of the frusto-conical shaft portion 4 to a portion of the right cylindrical shank portion 2. The flat portions 5 extend parallel with the longitudinal axis of the blank 1. Male threads are formed on the blank 1 by conventional thread rolling process on all surface of the frusto-conical shaft portion 4 and the cylindrical shank portion 2.
As male threads are formed to the tip end, it is difficult to align the axis of the screw with the axis of the hole of the work. The screw tends to incline to the axis of the hole, so that incorrect or incomplete female threads tend to be formed. The result may be lack of sufficient screw retaining force or crack of the work while or after the thread forming process. Effort to maintain correct aligned position of the screw is tedious and disturbs rapid screw forming which is required in e.g. a production assembly line.
Further, as shown in FIG. 3, at the head end 6 of the flat portion 5 a vertical shoulder 7 or stepped portion is formed between the outer periphery of the cylindrical shank portion 2 and the head end 6 of the flat portion 5. As shown in FIG. 4, a deep spoon-like recess 8 is formed by the thread rolling process as each thread passes on the flat portion 5 formed on the shank portion 2. A complete thread 9 is formed on the right cylindrical portion of the shank portion so that while thread forming to the hole of the work a, sudden increase of thread forming torque occurs when the complete thread 9 is engaged with the internal surface of the hole. Thus, smooth operation of thread forming is disturbed and the screw retaining force is not sufficiently strong. Further, as the flat portion 5 and the stepped portion 7 are formed by the cold extrusion process in a metal mould, heavy load must be applied to form the stepped portion 7, resulting in a decrease of service life and early damage of the metal mould.